Reflectance of zooplankton and its detectability by fishes

Few studies have investigated reflected ultraviolet and polarized light cues from zooplankton and how these optical properties affect their contrast to zooplanktivorous fishes. An optical setup was used to measure the reflectance of three zooplankton species. In conjunction with measures of photoreceptor absorptance and environmental irradiance, the contrast of zooplankton was estimated with respect to the visual systems of rainbow trout (Oncorhynchus mykiss) and zebrafish (Danio rerio). Antagonistic cone mechanisms involving either ultraviolet-sensitive cones or short-wavelength sensitive cones with middle-wavelength sensitive cones and long-wavelength sensitive cones resulted in the greatest contrast of Daphnia to both fishes. When modeling involved polarized irradiance, there were no significant differences in contrast as a function of polarization, likely due to the small sample size of zooplankton analyzed. Overall, the results corroborate previous findings suggesting that shorter wavelength cone mechanisms play crucial roles in enhancing zooplankton detection.